The present invention relates to GPS Navigation Satellite Systems and more particularly to systems which make use of the carrier phase of signals provided by GPS satellites.
GPS Navigation Systems include a constellation of satellites each of which provides a coded signal which may be picked up by radio receivers on the surface of the earth. Separate coded signals from a set of satellites may be processed by a receiver system for use in determining location as defined by latitude and longitude based on the code carried by the signals. The operation of GPS Systems in determining location based on coded signals received from satellites reflects the conventional functioning of such systems.
However, it has been found that the signals generated by GPS satellites may be used in other ways and in particular the carrier phase of the signals may be used in certain surveying applications. For example, a pair of stationary antenna/receiver combinations may be located at the ends of a baseline (whose length is required to be determined) and used to track and measure the phase of a GPS carrier signal arriving at their respective antenna locations. The situation may be described by Equation 1: EQU .phi..sub.A -.phi..sub.B =COS.theta.* .DELTA.X or EQU .DELTA..phi.=.DELTA..phi..sub.O +.DELTA..phi..sub.N =COS.theta.* .DELTA.X (1)
Where:
.PHI..sub.A =phase measured by antenna A at point A PA0 .PHI..sub.B =phase measured by antenna B al point B PA0 .DELTA..PHI..sub.O =measured phase difference PA0 .DELTA..PHI..sub.N =initial integer (phase) ambiguity in whole cycles PA0 .theta.=angle between the horizontal and the signal path from the satellite to point A (or B) PA0 .DELTA.X=distance between points A and B PA0 .delta..phi.=measured phase increment PA0 .delta.X=distance increment
in which the total phase delay may be seen as indeterminate due to the nature of oscillatory signals since an unknown number of whole cycles may be involved. This unknown quantity .DELTA..PHI..sub.N is commonly referred to as the initial integer ambiguity and can only be resolved by additional steps in a surveying procedure. For static surveying, the simplest manner of resolving the initial integer ambiguity is by observing carrier phase over a period of time. It is clear from Equation 1 that the two unknowns .DELTA..PHI..sub.N and .DELTA.X cannot be solved with one measurement equation at a single instant in time. However, with a second measurement .DELTA..PHI..sub.1 made at a different time T.sub.1, the measurement situation becomes two dimensional in nature as indicated by matrix Equation 2: ##EQU1## Where: the subscripts 0 and 1 indicate measurements at different points in time
and the distance .DELTA.X may be readily determined. In real life three-dimensional GPS static surveying, the amount of time necessary to obtain a solution is typically on the order of twenty to forty minutes for relatively short baselines.
It may be appreciated how the principles given above can be extended to kinematic surveying applications so long as the phase of the carrier signal is continuously tracked. Continuity in the carrier phase profile provides the user with an exact history of positional changes of a roving antenna provided that some sort of initialization procedure has been used to resolve the initial integer ambiguity prior to antenna displacement. The situation may be described by Equation 3: EQU .DELTA..PHI.-.DELTA..PHI..sub.N +.delta..PHI.=COS.theta.* .DELTA.X+COS.theta.* .delta.X (3)
where:
Since the variables .DELTA..PHI..sub.N and .DELTA.X should have already been solved from the initialization, this leaves only .delta.X which can be solved instantaneously from a single phase tracking measurement .delta..PHI.. Kinematic surveying in accordance with the above procedures constitutes the primary application of GPS carrier phase techniques since distances may be easily derived from the carrier phase tracking measurements as long as continuous carrier tracking is maintained after initialization.
In order to speed up the resolution of the initial integer ambiguity during initialization, an alternative technique may be employed called "antenna exchange" as first introduced by B. W. Remondi in his article "Kinematic and Pseudo-kinematic GPS" in the Proceedings of the Satellite Division of the Institute Of Navigation's First International Technical Meeting in Colorado Springs, Colo. in 1985. B. W. Remondi suggested that by moving one antenna to the location of the other the total phase delay can be solved just as effectively as waiting for satellites to move appreciably. However, since there is no way to physically merge two antennas that need to occupy exactly the same location, the most effective approach is simply to exchange or swap the locations of the two antennas at the ends of the baseline. Antenna exchange can therefore be interpreted as kinematic movement of one antenna by an amount equal to 2*.DELTA.X while the other is kept stationary. This situation may be described by Equation 4: EQU .DELTA..PHI..sub.O +.delta..PHI.=COS .theta.* (.DELTA.X-2*.DELTA.X)-.DELTA..PHI..sub.N (4)
Measurements before and after the antenna exchange result in two simultaneous equations as shown in matrix Equation 5: ##EQU2## which may be readily solved to determine the initial integer ambiguity .DELTA..PHI..sub.N and baseline distance .DELTA.X.
The foregoing discussion indicates how the phase of GPS signals may be used in determining distances in surveying applications and how antenna exchange may allow for rapid "initialization." However, surveying applications have appeared to be the limit of the usefulness of carrier phase information derived from GPS satellite signals.
It is an object of the present invention to provide a system adapted for determining spatial orientation based on phase measurements of GPS satellite signals.
It is a further object of the present invention to provide a system for determining direction and/or attitude using an antenna array including antennas which may be configured in different patterns and which are each adapted for receiving and measuring the phase of signals from a set of GPS satellites.
It is another object of the present invention to provide a system for determining spatial orientation using GPS signals which may be employed on large scale vehicles such as ships and which is accurate in operation, economic to implement and reliable in service.